Theoretical study of i-Process contribution to the post-AGB star IRAS 14325-6428

摘要: The post-asymptotic giant branch (post-AGB) star IRAS 14325-6428 exhibits a peculiar heavy-element abundance pattern, characterized by a high [Ba/La] ratio and an anomalous odd-isotope Ba ratio (f_odd,ba=0.25±0.08), which is difficult to explain via the classical slow neutron-capture process (s-process). In this work, we theoretically investigate this abundance pattern by considering intermediate neutron-capture process (i-process) nucelosyntheis. We employ a self-consistent computational framework. A stellar model representative of the progenitor (1.5 M_sun, Z = 0.003) is evolved with MESA to the thermally pulsing AGB phase to obtain intershell elemental abundances. These values are then used as input for i-process nucleosynthesis calculations performed with a one-zone network (NucNet Tools) at constant temperature and density. The observed heavy-element abundances are best reproduced by a model with neutron density N_n = 5×10^13 cm^-3 and neutron exposure τ = 1.4 mbarn^-1, followed by dilution with a scaled-solar composition. This model reproduces the high [Ba/La] and [Ba/Ce] ratios and yields a post-dilution odd-isotop fraction f_odd,ba = 0.51, consistent with the observational trend. The abundance pattern arises from the i-process path approaching the neutron magic number N = 82, where the flow slows and the material accumulates at 135I, which subsequently β- decays to 135Ba. This leads to enhanced Ba production and reduced synthesis of heavier species. These results, consistent with the star's multi-element classification as enriched in both s- and r-process elements, provide quantitative support for an i-process origin of the heavy elements in IRAS 14325-6428, likely triggered by a relatively short-lived proton-ingestion episode in its AGB progenitor.